Method of manufacturing an anisotropic ferromagnetic body



1953 F. G. BROCKMAN ETAL ,7 5

METHOD OF MANUFACTURING AN ANISOTROPIC FERRQMAGNETIC BODY Filed June 28, 1961 2 Sheets-Sheet 1 F1 E- T 4- E INVENTORS.

ZZZAJKQfl hw z wb m' AGEMZ' Dec. 17, 1963 BRQCKMAN ETAL 3,114,715

METHOD OF MANUFACTURING AN ANISOTROPIC FERROMAGNETIC BODY Filed June 28. 1961 2 Sheets-Sheet 2 4T1 I Gauss 20000 [6000 IQOOO 8000 4000 4000 8000 ZDOO [6000 20000 H, Oersteds Gauss 20000 I6000 IZOOO 8000 400 /4000 8000 IZDOO 16000 20000 2000 H, 0e minis INVENTORS. .HZQNKGBROCMN y mmimfisoy r United States Patent METHUD OF MANUFACTURING AN ANISO- TRGPIQ FERRGMAGNETIC RUDY Frank G. Brockman, Dohhs Ferry, and Kenneth E. Matteson, Mahopac, NY, assignors to North American Philips Company, line, New York, N.Y., a corporation of Delaware Fiied June 28, 1961, Ser. No. 120,377

12 Claims. (Cl. 252-625) Our invention relates to ferromagnetic bodies which are magnetically anisotropic and the method of making the same. In particular, the invention relates to a method of magnetizing uniaxial ferromagnetic materials to make bodies of them which are magnetically anisotropic.

As employed throughout this specification and claims, a uniaxial magnetic material is one in which particles of the material have a preferred, or easy, direction of magnetization along one axis. This direction may coincide with a crystal axis, if the material is crystalline; or it may coincide with an axis of the body related to its shape.

Permanent magnets have been made which are magnetically anisotropic by pressing permanent magnet powder in a magnetic field so that the particles of the powder align themselves with the magnetic field. The resultant body then is magnetically oriented in the direction of the magnetic field and is, therefore, magnetically anisotropic. -Fenromagnetic materials, the monocrystals of which have a preferred direction of magnetization, have also been formed into magnetically anisotropic bodies by placing the material in finely-divided form in a magnetic field while compacting the material into a body.

However, the problem of providing a field to magnetize a body of such material with a toroidal or annular shape particularly in the circumferential direction is rather difiicult because a circular field is required. This can be realized by passing a current of I amperes through a conductor which is surrounded by the material and problem a field H oersteds at radius of r centimeters from the center of the conductor.

Thus, for example, oriented ceramic magnetic materials such as those disclosed in United States Patent 2,762,778 can be made by subjecting a finely-divided powder consisting of crystals of MFe O (M being barium, strontium or lead) to a magnetic field of about 2000 oersteds while the material is compacted, usually by pressing. To make a toroid of this material and to magnetically orient the material in the circumferential direction, the material then would have to be placed in an annular die provided with a center arbor through which a magnetizing current of several thousand amperes would flow.

For example, to press toroids of this material with an outside diameter of 0.7 cm. and an inside diameter of 0.5 cm, a direct-current of 2500 amperes passing through the center arbor would be required, while toroids which have an outside diameter of 2.07 cm. and an inside diameter of 1.44 cm. would require currents as high as 7000 to 8000 amperes passing through the center arbor. This requires a source capable of furnishing large currents, for instance, a large bank of capacitors charged to high voltages with elaborate switch means to discharge the capacitors through the conductor.

It is an object of our invention to provide a new and novel method of making magnetically anisotropic uniaxial ferromagnetic materials in which large currents required for producing the magnetizing field can be obtained more simply from conventional power sources.

3,ll4,715 Patented Dec. 17, 1963 Another object of our invention is to simplify the method of making magnetically anisotropic ring-shaped or toroidal ferromagnetic bodies constituted of uniaxial ferromagnetic materials.

Still another object of our invention is to make a circumferentially magnetized toroidal body of a uniaxial ferromagnetic material.

These and further objects of our invention will appear as the specification progresses.

Quite unexpectedly we have found that it is possible to orient a uniaxial magnetic material using an alternating current flowing through a conductor. This discovery is all the more surprising because heretofore alternating currents have been used only to demagnetize magnets.

It was assumed heretofore that if an alternating current were used in an attempt to orient a powdered uniaxial ferromagnetic material, the particles would dance about and so be in a disorganized, unoriented state when the pressing operation was carried out. However, contrary to all expectations, it was found that a powdered uniaxial ferromagnetic material could not only be magnetized, but also magnetically oriented in the field produced by an alternating current flowing through a conductor.

The application of this discovery to the manufacture of, for example, a toroidal body of a uniaxial ferromagnetic material obtained by pressing the powdered material in a die provided with a center arbor and one or two moving punches was, therefore, of real interest in view of the large current requirements as noted above. Instead of a capacitor bank and elaborate switch means for generating the large currents necessary to fully magnetize the ferromagnetic material, we found that we could orient this material in a circumferential direction very easily by connecting the arbor through a transformer to a suitable alternating current power source.

Thus, in order to obtain the large currents required, a step-down transformer was employed, i.e. one which converts a voltage E on its primary winding to a lower voltage E on the secondary winding in accordance with the well 'known relationship:

Eigh E, N,

where N and N are the number of turns of the primary and secondary windings respectively. This has the effect of increasing the secondary current I in the same ratio to the primary current I since the power consumed by the transformer is negligible. Thus, for example, a 60 c.p.s. tnansformer with a single-turn secondary and with a turns ratio of 328 gave a peak secondary current: I =(l.41 R.M.S.) X328 where is the root mean square primary current and with 15 to 16 amperes R.M.S. flowing in the primary, a peak secondary current of about 6,500 ampcres was available, sufiicient to magnetize and magnetically orient one of the toroids madeof the material mentioned above.

In accordance with our invention, therefore, We place finely-divided uniaxial ferromagnetic material in a suitable die and through an appropriate portion of the die which is conductive we pass an alternating current of such magnitude that the material in it is oriented while simultaneously compacting the material in the die. Then, if desired, the compacted material may be sintered to form a highly-coherent body.

The invention is particularly applicable to the preparation of bodies made from Ferroxdure-like materials. Ferroxdure-like materials are uniaxial magnetic materials. fierroxdure-like materials have an inherent crystallographic anisotropy, i.e., there is one crystallographic direction which is the easy direction of magnetization and in the case of those materials, the easy direction is the direction of the hexagonal axis. Examples of such materials are disclosed in United States Patent 2,762,777, and have a composition M Fe O where M is one of the materials in the group consisting of barium, strontium, and lead. These materials are made by mixing together the oxides of one or more of the metals barium, stronitum, or lead and ferric oxide in the proportions of about 6 mols of ferric oxide to 1 mol of the other metal oxide and sintering these oxides together at a high enough temperature to promote a solid state reaction. Another material having an inherent crystalline anisotropy which we have also used is BaCo Ti Fe O i.e. a titani um-cobalt substituted Ferroxdure-like material.

The invention, however, is not limited to such oxidic materials. Other ferromagnetic materials possess uniaxial crystal anisotropy, for instance, the alloy of manganese and bismuth. Fine powders of manganese-bismuth may also be oriented in accordance with our invention. Materials which have shape anisotropy can also be oriented in accordance with the invention. Thus, elongated fine particles of, for instance, iron and iron-cobalt, and the like may also be oriented in accordance with our invention. -It is essential only that the material to be oriented be uniaxial and in powdered form.

The invention is not limited to the orientation of toroidal or annular bodies although it will be disclosed in connection therewith, but it is applicable to bodies of any desired shape.

The invention will be described in connection with the following examples which are illustrative thereof and the accompanying drawing in which:

FIG. 1 is a plan view of a toroidal body magnetically anisotropic in a circumferential direction made according to the invention;

FIG. 2 is a cross-sectional view showing the crystal structure of the magnetized body shown in FIG. 1;

FIG. 3 shows a portion of a die which may be used for producing a toroidal body which is oriented in a circumferential direction in accordance with the invention;

' FIG. 4 is an elevational view of the conductor with a single crystal which is aligned by the current in the conductor;

FIG. 5 is a sectional view of the conductor with a single crystal aligned relative thereto.

FIG. 6 is a hysteresis diagram of a magnetically anisotropic body made in accordance with the invention obtained in a direction parallel to the preferred direction; and

FIG. 7 is a hysteresis diagram of the same body obtained in the direction perpendicular to the preferred direction.

For purpose of illustration, a toroidal body magnetically anisotropic in a circumferential direction, is shown in FIG. 1, having basal planes of the crystals 2, as shown by the dotted lines, extending in a radial direction, the direction of magnetization being perpendicular to the basal planes of the material and, therefore, in the circumferential direction. This is shown more clearly in FIG. 2 which shows a cross section of the magnet in FIG. 1, the basal planes of the material being in the plane of the drawing.

The toroidal body 1, shown in FIG. 1, was made by placing finely-divided S1'Fe O 4, in die 3 provided with a center arbor 5 of a copper rod /2" in diameter connected to the secondary 7 of the transformer 8, the primary 9 of which was connected to a source of the alternating current, the turns ratio of which was chosen to be 328 so that with 15 amperes flowing in the primary, a secondary current which flowed in the rod of about 6,500 amperes was obtained. It is obvious that the parts of this die should be constructed of non-magnetic materials. It may also be advantageous to make some portions of non-conductive materials to prevent the short circuiting of the current flowing in the central arbor through the outer part of the die. (The contact resistance between the sliding parts of the die and the outer cavity, however, is usually great compared with the resistance of the arbor and so this precaution is not usually necessary.) With this current in the copper rod, a field of 1,300 oersteds at a radius of 1 cm. from the rod was obtained which was suflicient to orient the material. During the passage of current, the material was compacted by a plunger 6 and the compacted material, after removal from the die was sintered at about 1,300 C. for about two hours.

The orientation of the material in this body is shown in FIGS. 4 and 5 where the location of a single crystal (hexagonal) relative to the central arbor is seen. The basal plane 2 of the crystal is aligned radially with respect to the rod and since the easy or preferred direction of magnetization of the crystal is perpendicular to the basal plane, or in the direction of the hexagonal axis, the direction of magnetization is tangent to a circle coaxial with the rod, i.e. in a circumferential direction.

In order to demonstrate that our invention is not limited to the toroidal shape, a body was pressed in a die of square cross section, 1.4-3 cm. x 1.43 cm., using a powder of SrFe O Orientation of the powder was effected by applying an alternating magnetic field before and during pressing. The direction of this field was parallel to one of the cube edges and perpendicular to the direction of pressing. The frequency of the alternating field was 60 c.p.s. The pressed body was fired at about 1350 C. for two hours. After firing the dimension in the oriented direction had decreased, due to sintering, from 1.43 to 1.10 0111., whereas the dimension perpendicular to the oriented direction decreased from 1.43 to 1.25 cm. This difference in shrinkage upon firing is a well known characteristic of well oriented Ferroxdure-like materials.

Further indication of the high degree of orientation of this body is given by FIGS. 6 and 7. There are hysteresis loops of this body, shown as plots of the intrinsic induction, 41rI, versus the field strength, H. FIG. 6 was taken with the measuring field parallel to the oriented direction. This hysteresis loop has many of the characteristics of a single crystal measured in the magnetically preferred direction, that is, a rapid rise of the induction at low fields and the attainment of saturation at relatively low fields. FIG. 7 was taken with the measuring field perpendicular to the oriented direction. FIG. 7 has the characteristics of a uniaxial single crytsal measured in the direction of dimcult magnetization, viz, the induction rises slowly with increasing measuring field and saturation is not attained even at 18,000 oersteds field strength.

It should be clearly understood, of course, that the above examples are illustrative only and that the invention is not limited to orienting toroidal or rectangular bodies, or to Ferroxdure-like material but rather that it is applicable to bodies of any shape and to any uniaxial ferromagnetic material. Nor is the invention restricted to the use of 60 c.p.s. alternating currents, so that reasonable deviations from this frequency, both higher and lower, can be used. If the frequency is too high, difficulties will be encountered due to the losses in the magnetic material, since these losses are always greater at higher frequencies.

Thus, while we have described our invention in connection with specific embodiments thereof, other modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention which is defined in the appended claims.

What we claim is:

1. In the method of manufacturing a magnetically anisotropic ferromagnetic bzody the steps of subjecting a finely-divided uniaxial ferromagnetic material, the particles of which have a preferred direction of magnetization along a given axis, to the action of an alternating current magnetic field of sufiicient intensity to magnetically orient the particles in a direction parallel to the direction of the field and simultaneously compacting said material into a coherent body.

2. In the method of manufacturing a magnetically anisotropic ferromagnetic body the steps of subjecting a finely-divided uniaxial ferromagnetic material, the par ticles of which have a preferred direction of magnetization along a given axis, to the action of an alternating current magnetic field of suificient intensity to magnetically orient the particles in a direction parallel to the direction of the field and simultaneously compacting the material, and sintering the compacted material into a highly coherent body.

3. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction the steps of placing a finely-divided uniaxial material, the particles of which have a preferred direction of magnetization along a given axis, in a circumferential alternating magnetic field of sufficient intensity to magnetically orient the material in the circumferential direction and simultaneously compacting the material to form a coherent body.

4. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction the steps of placing a finely-divided uniaxial material, the particles of which have a preferred direction of magnetization along a given axis, in a circumferential alternating magnetic field of sufiicient intensity to magnetically orient the material in the circumferential direction and simultaneously compacting the material, and sintering the compacted material to form a coherent body.

5. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction the steps of placing a finely-divided uniaxial material, the particles of Which have a preferred direction of magnetization along a given axis, around a conductor carrying an alternating current producing a circumferential alternating magnetic field of sufiicient intensity to magnetically orient the material in the circumferential direction and simultaneously compacting the material to form a coherent body.

6. In the method of manufacturing a magnetically anisotropic body of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization the steps of, placing said material in finely-divided form in an alternating magnetic field of sufiicient intensity to magnetically orient the crystals in a direction parallel to the direction of the field and simultaneously compacting the material to form a coherent body.

7. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization the steps of placing said material in finely-divided form around a conductor carrying an alternating current producing a circumferential alternating magnetic field of sufficient intensity to magnetically orient the crystals in a circumferential field and simultaneously compacting the material to form a coherent body.

8. In the method of manufacturing a magnetically anisotropic body of a uniaxial ferromagnetic material consisting of hexagonal crystals having a preferred direction of magnetization and having a composition MFe O M being a metal selected from the group consisting of Ba, Sr, and Pb, the steps of, placing said material in finelydivided form in a field produced by an alternating current of sufficient intensity to magnetically orient the crystals, and simultaneously compacting the material to form a coherent body.

9. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization and having a composition MFe g g, M being a metal selected from the group consisting of Ba, Sr, and Pb, the steps of, placing said material in finely-divided form around a conductor carrying an alternating current producing a circumferential alternating magnetic field of sufiicient intensity to magnetically orient the crystals in a circumferential direction and simultaneously compacting the material to form a coherent body.

10. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization and having a composition BaFe O the steps of, placing said material in finely-divided form around a conductor carrying an alternating current producing a circumferential alternating magnetic field of sufficient intensity to magnetically orient the crystals in a circumferential direction and simultaneously compacting the material, and sintering the compacted material to form a coherent body.

11. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization and having a composition SrFe O the steps of, placing said material in finely-divided form around a conductor carrying an alternating current producing a circumferential alternating magnetic field of suflicient intensity to magnetically orient the crystals in a circumferential direction and simultaneously compacting the material, and sintering the compacted material to form a coherent body.

12. In the method of manufacturing a toroidal body magnetically anisotropic in a circumferential direction of a uniaxial ferromagnetic material consisting of hexagonal crystals each having a preferred direction of magnetization and having a composition PbFe O the steps of, placing said material in finely-divided form around a conductor carrying an alternating current producing a circumferential alternating magnetic field of sufiicient intensity to magnetically orient the crystals in a circumferential direction and simultaneously compacting the material, and sintering the compacted material to form a coherent body.

References Cited in the file of this patent UNITED STATES PATENTS 2,435,227 Lester Feb. 3, 1948 2,984,866 Schwabe May 23, 1961 2,984,871 Venerus May 23, 1961 2,999,271 Falk et al. Sept. 12, 1961 3,073,732 Hunsdiecker Ian. 15, 1963 FOREIGN PATENTS 758,320 Great Britain Oct. 3, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0, 3 114M715 December 17 1963 Frank G. Brockmen et alo It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l line 39 for problem read produces 3 column 3 line 7 a for "stronitum" read strontium column 4,, line 37 for "'There" read These line TO Signed and sealed this 29th day of September 1964a (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN THE METHOD OF MANUFACTURING A MAGNETICALLY ANISOTROPIC FERROMAGNETIC BZODY THE STEPS OF SUBJECTING A FINELY-DIVIDED UNIAXIAL FERROMAGNETIC MATERIAL, THE PARTICLES OF WHICH HAVE A PREFERRED DIRECTION OF MAGNETIZATION ALONG A GIVEN AXIS, TO THE ACTION OF AN ALTERNATING CURRENT MAGNETIC FIELD OF SUFFICIENT INTENSITY TO MAGNETICALLY ORIENT THE PARTICLES IN A DIRECTION PARALLEL TO THE DIRECTION OF THE FIELD AND SIMULTANEOUSLY COMPACTING SAID MATERIAL INTO A COHERENT BODY. 